According to industrial experiences, the direct recycling of used tyres, that is, the direct in-material re-utilization of the rubber material of used tyres is not increasing at a decisive rate in spite of the widespread efforts for recycling. One of the main reasons for that is that high-purity granulates (milled material) being practically free from metals, synthetic fibres and other contaminants, and being produced by conventional mechanical milling, as well as granulates having a small particle size required by direct re-use (i.e. in most cases having a particle size below 400 micron) are made available only by very few recycling companies. The other reason is that the widely applied mechanical milling processes involve milling the entire tyre, making the resulting rubber granulates a mixed-material substance as it will contain all kind of rubbers applied for producing the tyre, and it will also have different composition depending on the proportion of car and truck tyres used for the milling process.
In the methods known in the field, the problems related to the applicability of mixed mechanically produced rubber crumbs are attempted to be solved by subjecting the mechanically produced crumbs to different chemical and mechanical treatments. Accordingly, several different methods are known for producing regenerated material for rubber industry. However, applying these methods involves extra costs, and in many cases the methods were not proven to be economical.
These problems have contributed to the development of the industrial field of high-pressure liquid jet tyre milling and thus there exist several apparatuses adapted for producing rubber crumbs. Many of these known apparatuses and methods are disadvantageous from the aspect of economy due to the size inhomogeneity of the produced rubber crumbs and to their high specific energy demand.
In WO 2009/129906 A1 a process and a plant for the water jet disintegration of materials is disclosed. The objects set for the systems disclosed include, in addition to processing tyres of generally applied sizes, the milling of the elastomeric material of oversized tyres, rubber tracks, conveyor belts and cables applying ultra-high pressure (in excess of 6000 bar) water jets. The document disadvantageously lacks the discussion of the energy-related implications of the applied method, including the problems posed by the generation of milling heat which is a particularly significant phenomenon in the pressure range suggested by the document. Furthermore, the document does not disclose the appropriate fixing and supporting of the various workpieces during milling, and also does not handle the dewatering of the produced crumb slurry and the drying of the end product.
The technological concept according to WO 2009/068874 A1 is more complex than the above detailed solution, providing, in addition to describing the liquid-jet processing of different tyre types, one type of proposal for dewatering the crumb slurry and drying the end product. The objectives of the application also include the processing of oversized (among others, so-called off-the-road) tyres. However, a significant share of these tyres is not entirely of a steel-cord radial type (nevertheless they may comprise steel cord plies in the reinforcement under the tread). According to the document, together with the rubber material the synthetic fibre content of the carcass of the tyre is also milled by the high-pressure water jets, removal of which, depending on the material of the fibres (polyamide, polyester, aramid, etc.) may pose different problems. The presence of synthetic fibre contaminants in the rubber crumbs to be used as recycled material is extremely disadvantageous. Furthermore, the method for processing the tyres is not described in detail in the document. The method disclosed in the document has a further disadvantage, namely that use of abrasive materials is involved in multiple steps of the method, for the removal of which from the rubber material there is currently not known an efficient industrial solution. Besides that, the solution according to the document disadvantageously does not touch upon the problems of energy loss through heat generation during the milling process.
In addition to a comprehensive description of an industrial technological solution, the construction schematics of certain milling apparatuses is also disclosed in WO 2010/023548 A1. The document also does not concern itself with the subjects of the energy efficiency of the milling process and heat generation during milling, yet these effects are very significant in the pressure range (above 3000 bar) specified in the document.
As it can be observed, a recurring deficiency of known solutions is that they either fail to deal with the energy-related conditions of the milling process, or deal with it in an insufficient depth. During high-pressure water jet milling the liquid jet impacting against the surface to be milled has a very significant kinetic energy, resulting basically from the velocity of the liquid jet. According to literature data, this velocity is 630 m/s at a pressure of 2000 bar, while at a pressure of 3000 bar it may reach 780 m/s.
In contrast to the above cited solutions, in WO 2008/084267 A1 a theoretical discussion of the high-pressure (ultra high-pressure) liquid jet milling process is disclosed. According to the document, the energy-related conditions of the milling process are examined in relation to the kinetic energy represented by the high-pressure jet and the so-called tearing energy of the elastomer to be milled.
An apparatus capable of carrying out an ultra-high pressure liquid jet milling process is disclosed in WO 2010/007455 A2. The apparatus according to the document allows for mounting two tyres on a common shaft. During the milling process, the shafts carrying the tyres may be driven from outside the milling space. The tyres are secured to the shaft applying a mechanism supporting the tread of the tyre from the inside.
High-pressure water jet milling processes are disclosed also in WO 01/53053 A1, CN 202498654U, GB 2 339 708 A, DE 196 48 551 A1, CN 200988284Y and CN 101224609A, and in Hungarian patent applications P 11 00429 and P 12 00305. According to Hungarian patent application No. P 11 00429, the working fluid is separated in the course of the method from the produced crumbs, from which the technological heat is recovered by the help of a dedicated solution before recycling the working fluid.
Most of the known solutions have the common disadvantage that a significant amount of heat is generated during the milling process, which results in the intense warming of the working water slurry containing the rubber crumbs produced by the process. Re-cooling this fluid involves wasting energy in two different ways. First, the energy required for heating up the liquid jet is wasted, and second, the warmed-up liquid needs to be re-cooled later, which also requires extra energy.
Therefore, one of the most important problems that is not solved by the known solutions is that for producing crumb elastomer in an economical manner, the energy demand, and thus the energy consumption of the process have to be reduced. In light of the above cited known solutions, therefore, the need has arisen for providing a method and an apparatus for producing milled elastomer by means of which the—preferably homogeneous, fine-grained—milled elastomer can be produced more economically, more efficiently, and with a lower energy demand compared to known solutions.